Wireless Sensor Networks (WSNs) are networks of small and tiny lightweight nodes that are randomly deployed in a large area where it is not possible to monitor continuously. Some physical parameters such as pressure, temperature and relative humidity etc. are used for monitoring the same. Energy consumption is the most important and critical issues for WSNs. The paper classifies the routing protocols based on the basis of two criteria: layers and architecture. Further, a survey of 15 routing protocols are done with their comparison by considering the factors like energy, power consumption, latency, network life etc.